The present invention relates to a process for producing metallic mercury from mercurous chloride commonly known as calomel, an installation for carrying out this process, and to metallic mercury obtained in this way.
It is known that the mercury contained in sulfur-containing minerals, such as blende or pyrites, is volatilised during roasting of these minerals and that it is encountered in its elementary form in the sulfur-containing gases produced by the roasting. As these gases are then employed for the manufacture of sulfuric acid, this latter will contain the mercury thus entrained by volatilisation, which will then be encountered in fertilizers, or others products that enter into the food chain, in the manufacture of which the thus products sulfuric acid is employed. For this reason, the presence of mercury in sulfur-containing gases is undesirable and the content thereof needs to be limited in the roasting gases.
The most frequently used process for washing the gases is known under the name of the "Boliden Norzink process", taught for example in U.S. Pat. Nos. 3,849,537 and 4,233,274 and described in a large number of publications. In accordance with this process, the gases to be purified are washed in a tower in which a reaction take place between the metallic mercury of the gas and the Hg.sup.++ mercuric ions in solution in the liquid phase from washing; this produces a very slightly soluble mercurous compound Hg.sub.2 Cl.sub.2. The calomel thus producted can be decanted and extracted from the system. This process which allows 99.9% purification to be obtained gives satisfactory results but suffers from the drawback of giving rise to the production of calomel, the market outlets for which are extremely limited and which, moreover, because of its volatility, is not an ideal material for subsequent treatment in conventional mercury production plants. Moreover, the amount of calomel that can be stored is limited by statutory regulations.
The aim of the present invention is to provide a simple process that can be directly integrated into a mineral roasting process, for producing metallic mercury of greater than 99.999% purity, for which there are appreciable market outlets and which as the advantage of being able to be stored in a small volume by using suitable packagings.